1. Field of the Invention
The present invention relates generally to optical waveguides for the transmission of electromagnetic energy. The present invention relates more particularly to optical couplers for coupling optical fibers, and methods for making them.
2. Technical Background
Fiber lasers have many attractive properties that make them suitable for various industrial applications. Such properties can include one or more of good beam quality, easy thermal management, compact size, and good efficiency. Fiber lasers are therefore often preferred to conventional types of lasers, such as solid-state and gas lasers. Fiber lasers are able to produce optical output in the several kW range with excellent beam quality. Thus, these lasers can be used for macro-machining applications like welding and cutting of metal. Furthermore, fiber lasers lend themselves for operation with ultra-short pulses by a method of mode-locking, enabling them to be used in micro-machining applications as well.
As any laser, a fiber laser can include a gain medium, an optical resonator, means of coupling energy into the gain medium, and means of extracting light out of the optical resonator. The gain medium in a fiber laser can include a length of an optical fiber, the “active fiber.” Typically the core of the active fiber is doped with optically active atoms such as rare-earth atoms (e.g., Er or Yb). The optical resonator can be formed by surrounding the gain medium with mirrors that, when properly aligned with respect to the active fiber, force some of the light emitted by the active atoms to bounce between the mirrors through the gain medium and get amplified. The mirrors can be either bulk optical mirrors, or they can be directly fabricated into optical fibers. In the latter case they are usually fiber Bragg gratings (FBGs), but other fiber-based or free space mirrors can also be used. Fiber-based mirrors are attractive since they can be directly attached or spliced to other fibers with very low optical losses. The mirrors, or typically only one of the two mirrors, are made only partially reflective to provide a route for extraction of light out of the optical resonator. In fiber lasers, the extracted light can be further guided with a length of optical fiber close to the point of interest, such as the work-piece. The extracted light thus forms a beam of laser light that can be used in the final application.
The active fiber is typically an electric insulator. Thus, energy cannot usually be supplied to it directly in the form of electric power. However, the active atoms absorb optical radiation within certain wavelength ranges called their absorption bands. This property is utilized in fiber lasers by feeding or “pumping” the energy into the gain medium in the form of radiation. This radiation is called the pump radiation, and is usually generated by pump diode lasers, which preferably are fiber coupled. Thus, a fiber laser typically includes a pump coupler that couples the pump radiation into the active fiber. It can be desirable that the pump coupler has a signal feed-through whereby the laser signal can pass through the coupler with low optical losses. This property can be beneficial to the properties of the laser cavity, and is generally desired in industrial class fiber lasers.
There are a number of ways of making a pump coupler using free-space optics, but these can typically require painstaking alignment and can be very sensitive to vibration and thermal effects. All-fiber pump couplers are known, but these can be difficult to fabricate, and often induce unacceptable losses to the pump light and/or the fed-through signal. Moreover, it can often be desirable for the signal feed-through fiber to pass a signal having a given polarization; many existing coupler designs are incompatible with polarization-maintaining fibers or other polarization-maintaining optical components.
Accordingly, there remains a need for a polarization-maintaining coupler that addresses one or more of the drawbacks or deficiencies of the prior art.